Each of Exercises 61 through 68 involves either the chain rule for partial derivatives or the incremental approximation formula for functions of two variables.

An editor estimates that if x thousand dollars are spent on development and y thousand dollars are spent on promotion, approximately Q(x, y) = 20x^3/2y copies of a new book will be sold. Current plans call for the expenditure of $36,000 on development and $25,000 on promotion. Use calculus to estimate how sales will be affected if the amount spent on development is increased by $500 and the amount on promotion is decreased by $1,000.